This invention relates to thermoplastic resin compositions and more particularly is concerned with polycarbonate resin mixtures having improved impact strength, especially in thick sections, and good resistance to environmental stress crazing and cracking.
Aromatic carbonate polymers are well known commercially available materials having a variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenol)propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. Generally speaking, aromatic polycarbonate resins offer a high resistance to the attack of mineral acids, may be easily molded, and are physiologically harmless as well as stain resistant. In addition, such polymers have a high tensile and impact strength, (except in thick molded sections), and a dimensional stability surpassing that of other thermoplastic materials. However, in certain applications, the use of aromatic polycarbonate resins is limited because they exhibit severe environmental stress crazing and cracking. "Environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents such as, for example, gasoline, particularly high octance no-lead gasoline, acetone, heptane and carbon tetrachloride when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. The most significant effect is a loss in vital impact strength and also an increase in brittle-type failure. Contact with such solvents may occur, for example, when parts are used under the hood of automobiles, or near the gasoline filler ports thereof, or when solvents are used to clean or degrease stressed parts made from polycarbonate resins.
At present, no entirely satisfactory means is available for reducing environmental stress crazing and cracking of polycarbonate resins, although a variety of methods have been proposed.
Blends of aromatic polycarbonate with butadiene styrene and polyolefins are disclosed in European Patent Application No. 28753, laid open May 20, 1981. Among the properties disclosed for the blends is reduced sensitivity to stress cracking. General proportions of blend constituents are 80-96.5 weight percent aromatic polycarbonate, 1-10 weight percent polyolefin and 2.5-10 weight percent butadiene styrene polymer containing 30-90% butadiene or a graft copolymer of 80-10% mixture of 50-100% styrene and 0-50% acrylonitrile on 90-0 percent polybutadiene.
U.S. Pat. No. 3,239,582 discloses blends of 95 to 80 weight percent polycarbonate and 5 to 20 weight percent of an alkenylaromatic polymer or copolymer. The term "alkenyl aromatic" is defined and exemplified at column 2, lines 10-47. The blends were disclosed as having an improved melt viscosity compared with polycarbonate itself. No mention of improved resistance to stress cracking is made.
Still other modifiers have been proposed for impact strength improvement, but none of them provides optimum environmental stress crazing and cracking resistance--applicant's earlier filed commonly assigned U.S. patent applications, Ser. No. 238,643, filed Feb. 26, 1981, now abandoned; Ser. No. 343,949; filed Feb. 29, 1982, now U.S. Pat. No. 4,430,476; and Ser. No. 352,382, filed Feb. 25, 1982, now U.S. Pat. No. 4,444,949. Ser. Nos. 238,643 and 352,382 describe polycarbonates modified with a combination of a butadienestyrene block type copolymer, an acrylate core shell interpolymer and, optionally, an olefin/acrylate copolymer. Such compositions process well and are toughened, but there is no disclosure of significant solvent resistance and, as will be shown later herein, by themselves, the block type copolymers do not provide significant resistance to environmental stress crazing and cracking at relatively low and moderate levels, even in thin sections. Ser. No. 343,949 describes polycarbonate resins modified with a combination of the block type copolymers and a linear low density polyolefin resin. There is no mention that such modifier combinations will provide enhanced resistance to environmental stress crazing and cracking.